The present invention relates generally to the field of Magnetic Resonance Imaging (MRI) and Magnetic Resonance Spectroscopy (MRS), and more particularly, to a method and apparatus for gradient echo MR imaging to measure a T2 relaxation rate constant and/or to create T2 contrast images together with T1 or spin density and T2* weighted images, all from the same MR data set.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
One of the problems with acquiring T2 weighted images is that conventional techniques require a relatively long repetition time (TR) in order to achieve a pure T2 (proton density) image contrast. In other words, T2 weighted imaging is generally associated with long imaging times, as compared to T1 weighted imaging. T1 imaging is much faster since such imaging is achieved by shortening the repetition time.
Measuring T2 relaxation time in MRI or MRS, as well as creating T2 contrast in MRI typically requires use of a version of spin echo (SE) pulse sequences. These sequences can differ in how signal excitation, phase encoding, and acquisition are combined together to form a pulse sequence. Different pulse sequences result in different image acquisition times, signal-to-noise (SNR) ratios and image contrasts. The most frequently used sequences for clinical diagnostic applications are traditional multi-slice two-dimensional Fourier Transform SE sequences and single-slice fast spin echo sequence techniques such as the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al. in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled xe2x80x9cRARE Imaging: A Fast Imaging Method for Clinical MR.xe2x80x9d The former usually requires long acquisition times, on the order of minutes, and the latter represents a fast imaging approach where only a single slice can be acquired in a subsecond time scale. The common attribute of all SE techniques is the presence of refocusing RF pulses. Most often, 180xc2x0 RF pulses are used. The major reason to use SE techniques stems from the refocusing nature of the 180xc2x0 RF pulses, which substantially reduces the influence of unwanted magnetic field inhomogeneities on MRI signals. These approaches can suffer from motion artifacts and imperfections in slice profiles resulting from the presence of the 180xc2x0 refocusing pulses. More importantly, however, SE techniques suffer from restrictions on RF power deposition. This factor substantially limits the clinical application of these techniques for high-field MR imaging.
It would therefore be desirable to have a method and apparatus capable of creating T2 contrast images using gradient echo imaging to acquire T2 contrast images in a time that approximates T1 imaging. It would also be advantageous to be able to use the same MR data that is used to construct the T2 weighted images to construct T1 or spin density images, and T2* weighted images.
The present invention provides a method of measuring T2 relaxation time constant in MRS and MRI, and creates T2 image contrast in MRI using gradient echo imaging that solves the aforementioned problems. In accordance with the present invention, the influence of magnetic field inhomogeneities on MRI signals can be removed by making use of a unique signal post-processing procedure in combination with a specially designed MRI pulse sequence without the use of refocusing RF pulses.
This technique is based on multi-gradient-echo approach and allows obtaining a T2 contrast images in MRI without using any type of SE techniques. Through use of the present invention, both T2 and T1 or spin density contrast images can be obtained in a single scan by adjusting flip angle and repetition time (TR). Further, images corresponding to long gradient echo times in a multi-gradient-echo train can be used to form T2* contrast. Therefore, this technique allows obtaining T2, T1 or spin density, and T2* weighted images in a single scan. All such images will also be naturally co-registered which is particularly advantageous for certain clinical applications.
The present invention can be implemented using either two dimensional or three dimensional pulse sequencing and since relatively low flip angles are used, the present invention requires substantially less RF power than SE acquisition techniques. The low flip angles also provide improved slice profiles as compared to traditional SE acquisition techniques. The present technique allows for fast imaging with repetition times (TR) on the order of 25 msec. on a typical clinical scanner, which promotes breath-held scanning to reduce motion artifacts. A half-Fourier approach can also be applied to increase the speed of the acquisition further.
Application of the present invention provides images that are largely insensitive to RF field inhomogeneities and also allows separation of water and lipid contributions in an image. A magnetization preparation block can also be used to suppress lipid or CSF signals, or to enhance T1 contrast.
A method of acquiring MR images, according to the present invention, includes determining a nonlinear function of gradient echo time to offset magnetic field inhomogeneities. Multiple sets of MR data are acquired from a series of read-out gradients in a pulse sequence. The invention also includes fitting the MR data to an equation that includes the nonlinear function, and then creating T1 or spin density images, and T2 images using the results of the fitting step. The invention can also be used to create T2* images using the same data set.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to rapidly acquire T2 weighted images that includes a magnetic resonance imaging (MRI) system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire the MR images. The MRI apparatus also has a computer programmed to acquire multiple sets of MR data from a series of read-out gradient pulses in a pulse sequence and determine signal intensity for each MR data. The computer then fits the MR data to a signal magnitude equation that includes a nonlinear function, and then reconstructs T2 weighted MR images that are substantially free of magnetic field inhomogeneities.
In order to reconstruct the T2 weighted images, the method includes a computer program for use with an MRI apparatus that includes instructions which, when executed by a computer, cause the computer to apply a pulse sequence with a train of gradient read-out pulses and acquire MR data during the train of gradient read-out pulses. The program determines a nonlinear function of gradient echo time based on the object scanned and the physical characteristics of the MR apparatus. The program then fits signal magnitudes of the MR data to a signal magnitude equation and reconstructs MR images using the results of the fit wherein the MR images reconstructed can include not only T2 weighted images, but also T1 or spin density images. T2* images can optionally be reconstructed using the same data set.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.